Direct viewing storage cathode ray tube circuit arrangement



ERECT VIEWING STORAGE CATHODE RAY TUBE CIRCUIT ARRANGEMENT A. M, ADAMFiled July 18. 1966 ADJU TABLE 05m) PULSE :auece 8 ouus 71151.5 PULSEsauAcE FIGZZ sa /Rae 0F VARY/N6 SCREEN POTENTIAL.

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: lNvem-rota flaw/m, 2 1;, Adams; ATTORNEY United States Patent 33,235/65 US. Cl. 31512 Int. Cl. H013 29/ 41 6 Claims ABSTRACT OF THEDISCLOSURE A direct viewing storage tube arrangement including a viewingscreen, a storage target electrode, a reading electron gun for scanningthe target electrode for writing information thereon in the form ofstored charges and means for providing a repetitively pulsed flood beamfor the production of visible images on the viewing screen. Decay pulsesare repetitively applied to the target electrode during alternate pulsesof the flood beam and the E. H.T. potential applied to the fluorescentscreen in varied between a minimum potential corresponding in time tothe applied decay pulses and a maximum potential corresponding in timeto the remaining flood beam pulses, corresponding to the production of avisible image.

This invention relates to circuit arrangements incorporating directviewing storage cathode ray tubes and to methods of operating suchtubes. More specifically the invention relates to methods ofcontinuously operating direct viewing storage cathode ray tubes(hereinafter termed, for brevity, direct viewing storage tubes) and tocircuit arrangements for carrying those methods into effect. The objectof the present invention is to provide improved methods of continuouslyoperating direct viewing storage tubes and improved circuit arrangementsfor carrying out those methods which shall be better suited for therequirements of such equipments as surveillance radars, where gooddisplays, with persistence times of 2 minutes or more, are usuallywanted, than are known methods and arrangements.

By continuous operation of a direct viewing storage tube is meant thatform of operation in which information to be displayed-for exampleinformation provided by signals derived by a surveillance radarisprogressively written in the form of an electrical charge image on thestorage target of the tube and this image is continuously decayed orattenuated. The at present usual method of achieving such attention ordecay is by simply applying periodically repeated short pulses,hereinafter termed decay pulses to the normally provided backingelectrode of said target. These may be a single train of applied decaypulses or two (or more) trains of pulses of different durations andamplitudes. Thus, a train of short duration high amplitude pulses and atrain of longer duration lower amplitude pulses may both be applied asdecay pulses. For clarity in explanation it may be pointed out thatcontinuous operation is in contra-distinction with what is usuallycalled one shot operation in which information is written on the storagetarget during a scan thereof by the signal modulated writing gun of thetube, the charge image thus obtained being stored indefinitely on thetraget and viewable at any time on the fluorescent screen of the tube byswitching on the flood gun. In one shot operation, when a stored chargeimage is no longer required and is to be replaced by another, a longerasing pulse, customarily of about 1 second duration, is applied to thebacking electrode of the target. In continuous operation, on the otherhand the charge image is attenuated as distinct from being erased. Theattenuation being Patented Feb. 11, 1969 effected, in known present daypractice, by periodically applying very short pulses to the targetbacking electrode.

Serious difficulties and limitations are encountered in securingsatisfactory continuous operation of a direct viewing storage tube byknown methods. The storage elements are charged negatively by the decaypulses and receive positive charges by the action of the writing beamand from positive ion bombardment from residual gas molecules.Equilibrium is reached when the long term averages of negative andpositive charging effects are equal. Variations in positive ion chargingcause excessive shifts in this equilibrium state when operation at lowdecay rates is attempted and in practice, with most tubes, the practicallower limit for the decay rate is that which gives a persistence of onlyabout 30-60 seconds. If a lower decay rate is used the defect termedcharge spreading (blurring of the stored image) is apt to occur as aresult of the locally increased rate of positive-ion charging, which mayreach a value great enough to over-ride the decay action. Thepersistence time of 3060 seconds is, however, insufficient in many casese.g. in the case of a surveillance radar, where, as already stated, aradar display with a persistence time of 2 minutes or more is commonlyrequired.

Again, with known methods of continuous operation the decay pulses tendto degrade the black level of the viewed image, for the screen flashesto full white during each pulse. In practice this defect may not beserious because of the low duty ratio and high repetition rate of thedecay pulses usually employed. However, even if not unacceptably seriousin itself, it does restrict the decay pulses which can be used.Although, for other reasons, the use of a low amplitude high duty-ratiodecay pulse train might be preferred, the greater degradation of blackproduced by such a train as compared with a high amplitude, lowduty-ratio decay pulse train may prevent the adoption of the former. Thepresent invention seeks to overcome the foregoing defects andlimitations.

According to the invention in its broadest aspect, the flood beam of adirect viewing storage tube having decay pulses applied to its targetbacking electrode to obtain continuous operation of the tube is pulsed.

In one way of carrying out the invention successively occurring timeperiods during which the flood beam is effectively pulsed on areemployed for the decay phase and for the viewing phase of operation,said phases alternating with one another.

According to a feature of this invention apparatus for continuouslyoperating a direct viewing storage tube comprises means for writinginformation in the form of a charge image on the storage target of thetube, means for applying periodic decay pulses to said target toattenuate the charge images stored thereon and means for translating thesaid charge images into visible images on the viewing screen of the tubein pulsed fashion by means of a pulsed flood beam which intermittentlyfloods the target and passes therethrough to said screen.

The duty-ratio of the pluse flood beam (i.e. the pulse duration for agiven pulse repetition frequency) may be adjustable to providebrightness control.

By means of the invention it is possible to reduce the level ofdecayerase to one corresponding to a persistence time several times aslong as that which would be permissible when using the customary knownmethod of continuous operation without causing serious charge spreadingor blurring of the charge image. Thus if the flood beam is pulsed with aduty ratio of (say) 1 to 10, charge spreading, as compared to that whichoccurs when the flood beam is unmodulated, as in normal present daypractice, is reduced by a factor of 10. Although the invention involvesa comparative reduction of brightness of the viewed image (approximatelyin proportion to the duty-rates of the flood beam pulsing), in practicean image bright enough to be viewed in daylight is readily obtainable.

The flood gun conditions during on conditions of flood beam pulsing are,of course, those normally specified by the manufacturers for the tubeconsidered. The effective extinction of the Hood beam between pulses canbe effected in any of a variety of diflerent ways: for ex ample byreducing, in pulsed fashion, the potential of one or more of thenormally provided collimating electrodes of the tube; or, in the case oftubes provided with a space-charge grid in front of the flood gun, bytaking this grid to a negative potential; or by taking the normallyprovided backing electrode of the target to a potential of, say, tovolts more negative than the normal value; or (equivalently) by takingthe cathode of the flood gun to a potential of, say 5 to 10 volts morepositive than the normal value. It will be appreciated that, in the lastmentioned two cases, the flood beam will not be physically cut off butwill be diverted to the collector mesh. From the viewpoint of thepresent invention, however, this may be regarded as the equivalent ofcutting off the flood beam.

A further improvement is obtained if the E.H.T. applied to the screen ofthe tube is reduced to a low value or to zero, during a proportion ofthe flood beam pulses, all decay pulses being confined to periods of lowE.H.T. Thus, for example, the E.H.T. may be varied so that it is aminimum at the times of alternate flood pulses and a maximum at thetimes of the remaining flood pulses. If this is done, degradation of theblack level of the viewed image by decay pulses can be avoidedaltogether.

The decay pulses may be variable in amplitude and/or in duration.

The invention is illustrated in and further explained in connection'with the accompanying drawings in which FIGURE 1 is a much simplifiedschematic representation of one embodiment of the invention and FIGURE 2is an explantory graphical figure.

Referring to FIGURE 1 the tube therein represented is as known per seand comprises with an evacuated envelope 1 a fluorescent screen 2 on thelarge end wall of the envelope, a storage target or mesh 3, and acollector mesh 4. 5 represents a flood gun system for flooding thestorage target with electrons to translate a stored electrical chargeimage thereon into a corresponding visible image on the screen 2. In theneck of the tube is a writing gun 6 for scanning the target 3 throughthe mesh 4 and producing stored charge images thereon in well knownmanner. The tube is provided, on the walls of its enlarged bulb portion,with the customary electrodes 7 for collimating the flood gun electrons.

In the illustrated embodiment the flood beam from the gun is pulsed bypulsing one of the collimating electrodes though any other method ofpulsing the beam, such as those hereinbefore described, may be used.Block 8 represents a source of pulses each of which, when applied,permits the flood beam to flood the target 3 through the collector mesh.Between pulses the flood beam is efiectively cut off. The arrow on block8 indicates that the pulses are of adjustable duration.

Block 9 represents a source of decay pulses which, as indicated by thearrow, may be adjustable in amplitude and/or duration, and which areapplied to the target 3. There may be more than one train of applieddecay pulses, e.g. a train of short duration high amplitude decay pulsesand a train of longer duration lower amplitude decay pulses. Such trainscould be obtained from a plurality of pulse generators such as generator9, operating at different subharmonics of the frequency from generator8.

Block 10 represents a source of periodically varying E.H.T. applied tothe viewing screen 2.

FIGURE 2 illustrates the operation graphically. The

pulses in line (a) are two successive pulses from block 8 and representperiods when the flood beam is on. The dotted lines indicate that thepulse widths are adjustable. As will be seen the first of the two pulsesof line (a) occurs in a decay phase of operation and the second in aviewing phase.

Line (b) of FIGURE 2 shows in full line a decay pulse applied from block9. The decay pulses can be varied in amplitude, duration or timing (solong as they occur in decay phases) as is indicated by the dotted linepulse shown alongside the full line pulse in line (b) and, as alreadystated, a plurality of trains of decay pulses of different durations andamplitudes may be applied.

Line (0) shows the varying E.H.T. wave applied from block 10 to thescreen. As will be seen the E.H.T. is zero, or approximately so, duringthe decay phase, and is of approximately normal working value during theviewing phase.

The periodicity of flood beam pulsing may be adjustable if desired butif it is the periodicity of the decay pulses should be adjustedtherewith to ensure that the decay pulses and flood beam pulses occur inthe correct relationship illustrated by FIGURE 2, i.e. the decay pulsesmust occur within the on period of the flood beam pulses.

I claim:

1. A direct viewing storage tube arrangement including a tube having adisplay producing viewing screen, a storage target, means for writinginformation on the target by the production of a charge image thereon,means for applying periodically recurring decay pulses to said targetfor attenuating the charge images stored thereon, means for applying arepetitively pulsed flood beam of electrons to said target to producevisible images on said viewing screen, and means for applying to saidviewing screen a potential repetitively varying between a maximumpotential corresponding in time to the production of images on saidviewing screen by the flooding thereof and a minimum potentialcorresponding in time to the application of decay pulses to said targetby said means for applying periodically recurring decay pulses.

2.The arrangement according to claim 1 wherein said means for applyingperiodically recurring decay pulses to said target comprises means forproducing said decay pulses duringalternative applications of saidrepetitively pulsed flood beam to said target to provide visible imageproduction during the remaining alternative applications of saidrepetitively pulsed flood beam to said target.

3. The arrangement according to claim 1 wherein said means for applyinga repetitively pulsed flood beam is adjustable for altering theduty-ratio of said pulsed flood beam for controlling the brightness ofthe visible image.

4. The arrangemeent according to claim 1 wherein said means for applyinga repetitively pulsed flood beam comprises means for periodicallystopping the production of said flood beam.

5. The arrangement according to claim 1 further comprising a collectormesh within said tube, said means for applying a repetitively pulsedflood beam periodically providing diversion of said flood beam to saidcollector mesh.

6. The arrangement according to claim 1 wherein said means for applyinga repetitively varying potential to said viewing screen comprises meansfor applying said mini mum potential and maximum potential duringalternative applications of said pulsed flood beam to said target.

References Cited UNITED STATES PATENTS 2,931,938 4/1960 Patrick et al.315-12 3,277,333 10/1966 Williams et a1. 3l512 RODNEY D. BENNETT, 111.,Primary Examiner.

C. E. WANDS, Assistant Examiner.

